How to Make Buildings Resistant to Hurricanes
What do we mean by a Hurricane?
The earth has an atmosphere which extends about 600 miles (1000 km) high, which is dense near the surface but gets thinner as it gets higher. 80 % of the air is packed into the bottom 7 miles (11 km). Most of the weather phenomena happen in this 80 % of air. When the Earth spins, it tends to induce movement of the atmosphere. Close to the Equator, the tendency is for the air to move westward: from Africa towards the Caribbean and the Americas; and from Central America towards South East Asia and Australia; and from there back towards Africa and the Indian sub continent.
Further away from the Equator, the return trip for this moving air goes back to the East, in 4 streams. These are at high level, at about 30 degrees and 60 degrees North and South of the Equator. These are very high speed air streams about 7 miles up, called the Jet Streams. The change of direction, from West near the Equator to East further away, induces a little spin to the air masses near the Equator.
The earth is warmed by the sun, more in the Northern hemisphere from April to October, more in the Southern hemisphere from October to April; and more within 7 degrees to 20 degrees North or South of the Equator. Although Ocean Currents move the water around, the tendency is for water near the equator to warm up during the summer months. As the water gets warmer, it can pass more heat to the air above it; and more water evaporates into this air. Air that is being heated becomes less dense and tends to rise.
The varying patterns of currents and depths of water and areas of continent mean that the movement of air masses, to the West near the Equator, back to the East within the high, cold Jet Streams, is not smooth but erratic. Where water is wide but shallow, and has little current from colder areas, the temperature difference between the water and the cold upper air can become greater; and it is temperature difference, as well the amount of hot material, that powers a heat engine. It is probable that Global Warming is increasing the amount of heat in the sea, and temperature differences.
Now and then conditions get right for the formation of a tropical storm. The warm water heats the air and its starts to rise, and in doing so, the spiralling tendency it has is increased. The warm air will be very moist; it starts to condense and in doing so, gives up its ‘heat of evaporation’, the heat the sun applied to the water to make it evaporate in the first place. As the warm air spiral goes up into colder air, it becomes, relative to the surrounding cold air, even warmer and lighter, so it rises and spirals faster.
The swirling air, being lighter, weighs down less on the air below, sucking in more warm moist air from the surrounding sea. Sometimes, the Eastward flow of a Jet Stream can catch the top of the spiral and drag air away from it, reducing the pressure in the spiral even more. This enormous heat-engine, powered by the heat in the sea, is a tropical storm.
In addition to the violent wind, the very low air pressures have another trick: they can suck the sea-water towards the core. The sea level can go up to several feet above the normal level. Naturally, the extreme winds whip this extra-high surge of water into extra-high waves. When this surge, and the waves in it, hit a coast-line, the effects on flood defences and on buildings can be more serious than the damage from the wind. The storm has one more trick: all the water in the moist air that is sucked up has to come down somewhere.
At first, the air is going up so quickly that condensing water cannot fall down, but is instead carried upwards. Droplets splash together, becoming bigger and bigger; until they are thrown out of the spiral and can fall; or until they become heavy enough to drop through the upwards rush of air. The result is intense rainfall. If the droplets go high enough, big drops can freeze into hailstones (releasing the heat of melting, again helping to power the storm), which can grow to the size of footballs before they drop.
The storm is a swirling mass of cloud, which can be up to 600 miles (1000km) across. At its centre is the ‘eye’, a spinning tube of clear air whose centre can be slow-moving; but the sides of this eye can spin, spiralling upwards, at enormous speeds, pumping up the moist air. How intense the storm becomes depends on a combination of factors. The right position; a big enough, warm enough sea; the conditions in the upper atmosphere; all contribute to the size and ferocity of the storm.
Every ocean in the world, from 7 degrees to 20 degrees from the Equator, can produce these storms (except, oddly, the South Atlantic, where they never occur, perhaps due to the ocean currents, which do not allow the build up of big enough, warm enough, areas of sea). These storms are carried along, fairly slowly, by the air circulation in the atmosphere. The storm dies out only when its heat source is removed, when its core goes over land; or goes over cooler water.